Campbell Scientific 107 107-L Temperature Probe - Page 16

Operation

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Model 107 Temperature Probe 8. Operation 8.1 Sensor Schematic FIGURE 8-1. 107 thermistor probe schematic 8.2 Measurement and Output Linearization Campbell Scientific dataloggers measure the 107 probe thermistor and convert the result to temperature. With reference to the previous FIGURE 8-1, 107 thermistor probe schematic, a precise excitation voltage is applied at the Vx line and the voltage drop across the 1 kΩ resistor is measured at the Vs line. The ratio of measured voltage (Vs) to excitation voltage (Vx) is related to thermistor resistance (Rs), and the 1 kΩ ohm and 249 kΩ fixed resistors as described in the following equations: Vs/Vx = 1000 / (Rs + 249000 Ω + 1000 Ω) Solving for Rs: Rs + 250000 Ω = 1000 • (Vx/Vs) Rs = 1000 • (Vx/Vs) - 249000 TABLE 8-1, 107 Measurement Details, and TABLE 8-2, 107 Temperature Calculation, describe how measurement results Vs/Vx and Rs are converted to temperature by Campbell Scientific dataloggers. 10

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Model 107 Temperature Probe
8.
Operation
8.1
Sensor Schematic
FIGURE 8-1.
107 thermistor probe schematic
8.2
Measurement and Output Linearization
Campbell Scientific dataloggers measure the 107 probe thermistor and convert
the result to temperature.
With reference to the previous FIGURE 8-1,
107
thermistor probe schematic
, a precise excitation voltage is applied at the Vx
line and the voltage drop across the 1 kΩ resistor is measured
at the Vs line.
The ratio of measured voltage (Vs) to excitation voltage (Vx) is related to
thermistor resistance (Rs), and the 1 kΩ ohm and 249 kΩ fixed resistors
as
described in the following equations:
Vs/Vx = 1000 / (Rs + 249000
Ω
+ 1000
Ω
)
Solving for Rs:
Rs + 250000 Ω = 1000 • (Vx/Vs)
Rs = 1000
•
(Vx/Vs) –
249000
TABLE
8-1,
107 Measurement Details,
and TABLE 8-2,
107 Temperature
Calculation,
describe how measurement results Vs/Vx and Rs are converted to
temperature by Campbell Scientific dataloggers.
10